The present invention relates to an oxidative wastewater treatment process using a catalyst under a mild condition; and to the catalyst suitable for use in said treatment.
Industrial wastewaters generated at various plants often comprise toxic pollutants which cannot be removed by a simple biological process based on activated-sludge treatment. They are therefore treated, e.g., by a chemical process, either alone or in combination with a biological treatment process, to meet the discharge criteria of the effluent.
A chemical wastewater treatment process uses a chemical reagent that may oxidize, reduce, complexe or hydrolyse polluting materials present in the wastewater and a typical conventional chemical treatment method uses an oxidizing agent, e.g., hydrogen peroxide, ozone, permanganates, hypochlorites and the like. However, this conventional oxidative process suffers from the problem that the reactivities of the oxidizing agents with the pollutants are such that a severe reaction condition, i.e., a high temperature/pressure condition is usually required. Moreover, the conventional oxidative processes have additional problems, e.g, in the alkaline chlorination method for removing cyanides from waste water, chlorine may react with other components in the waste water to produce chlorinated compounds which may be toxic and not easy to remove. Accordingly, there have been a number of efforts to develop methods which are not hampered by such problems. For example, a catalytic method to oxidatively remove toxic pollutants from waste water at 250xc2x0 C. and 10 to 70 atm was reported in Jpn. Catalyst, 35(5), 289 (1993). However, this method employs a severe condition, and is thus not economical.
Further, a method to increase the reactivity of the pollutants by irradiating UV together with an oxidizing agent has recently been developed (see Ollis. D. F., Environ. Sci. Technol. 26, 313, 1992). However, this method also requires a high treatment cost.
Furthermore, U.S. Pat. No. 5,192,452 discloses a method for the waste treatment by the use of a catalyst comprising a first component selected from the oxides of titanium, silicon, aluminum and zirconium and a second component selected from Mn, Fe, Co, Ni, Ce, W, Cu, Ag, Au, Pt, Pd, Rd, Ru and Ir, or a sparingly water-soluble compound of the metal, with ozone, for the purpose of deodorization, sterilization and decolorization. However, the catalyst used in this method has a relatively low activity.
Accordingly, there has existed a need to develop a catalytic process which is more effective in oxidizing pollutants in wastewater under a mild condition, e.g., at ambient temperature and pressure.
It is, therefore, a primary object of the invention to provide an efficient catalytic process for treating wastewater with an oxidizing agent under a mild condition.
It is another object of the invention to provide a heterogeneous catalyst suitable for use in said catalytic process.
In accordance with an aspect of the present invention, there is provided a catalytic process for treating a wastewater stream with hydrogen peroxide, air, oxygen or ozone at ambient temperature and pressure.
In accordance with another aspect of the present invention, there is provided a catalyst for oxidative wastewater treatment, which comprises an element selected from the group consisting of transition metals, alkali metals, alkaline earth metals and a mixture thereof supported on the specified metal oxide.
In accordance with the present invention, an industrial wastewater containing various toxic pollutants can be treated economically and efficiently by way of using an oxidizing agent in the presence of a specified catalyst under a mild condition, e.g., at ambient temperature and pressure.
The process of the present invention is suitable for treating industrial wastewaters generated in petrochemical and refinery processes; coal chemical processes; and other processes to make agricultural chemicals, polymers, dyes, glasses, electric and electronic articles, paper products, textiles, machines and synthetic leathers; and the like.
Representatives of the compounds to be removed from industrial wastewaters in accordance with the present invention may include: phenolic compounds, e.g., phenol, monochlorophenol, dichlorophenol, trichlorophenol, tetrachlorophenol and pentachlorophenol; chlorinated aliphatic compounds, e.g., chloroform, carbon tetrachloride, 1,2-dichloroethane, 1,1,1-trichloroethane, tetrachloroethane, 1,2-dichloroethylene, trichloroethylene and perchloroethylene; benzene and chlorinated benzenes, e.g., monochlorobenzene, dichlorobenzene, trichlorobenzene, tetrachlorobenzene and hexachlorobenzene; alcohols, e.g., methanol, ethanol, propanol, butanol, pentanol, hexanol and cresol; toluene; xylene; acetonitrile; dipropylformamide; mercaptans; methylethylketone; nitrobenzene; organic phosphor compounds; phthalimides; quinolines; cyanides; and the like.
Representatives of the oxidizing agent which may be used in the present invention include air, oxygen, hydrogen peroxide, N2O and ozone, among which air and oxygen are preferred. The oxidizing agent may be employed in an amount suitable for lowering the COD (chemical oxygen demand) value of the waste water to a desired level.
The oxidation is preferably carried out in an aqueous solution, although an organic solvent may also be employed for a specific purpose, e.g., for a selective oxidation. When air or oxygen is employed as the oxidizing agent, it is preferable to conduct the oxidation at an operating pressure of 5 atm or higher.
In accordance with the present invention, a heterogeneous catalyst is employed together with an oxidizing agent. The catalyst is prepared by impregnating a support material with an aqueous or organic solution containing a metallic element selected from the group consisting of transition metals, alkali metals, alkaline earth metals and a mixture thereof.
The metallic element suitable for use in preparing the inventive catalyst may be Group IA elements such as Li, Na and K, Group VIII elements such as Ru, Co, Fe, Pt, Pd, Ni, Co and Rh, Group VII elements such as Mn, Group IB elements such as Cu and a mixture thereof; preferably, Ru, Co, Fe, Cu, Mn and a mixture thereof. The metallic element may be in a form soluble in water or in an organic solvent, and a chloride or nitrate thereof may be employed in an amount ranging from 0.0001 to 100% by weight; more preferably 0.01 to 50% by weight; and most preferably 0.1 to 10% by weight, based on the amount of the supporting material used.
The support material may be an oxide of Mg, Ca, Sr, Ba, Ge, Sn, Pb, a Lanthanoid element, an Actinoid element or a mixture thereof. Preferably, the support is MgO, CaO, BaO, PbO, SnO2 or a mixture thereof.
The preferred catalyst is a transition metal catalyst supported on MgO, and the most preferred catalyst is Fe/MgO.
The catalyst of the present invention may be prepared from the material impregnated in accordance with the embodiment of this invention as described above by drying and calcining at a temperature ranging from 200 to 600xc2x0 C., preferably 400xc2x0 C. for a period ranging from 0.1 to 24 hours, preferably 2 hours. The catalyst may be in the form of powders, pellets or granules. In the present invention, the waste water is preferably treated at a flow rate ranging from 0.001 to 1,000 LHSV (liquid hourly space velocity: volumes of the waste water fed per hour per unit volume of the catalyst).
The wastewater treatment process of this invention may be practiced by using any one of the known reactor systems, e.g., a fixed bed reactor, a fluidized bed reactor, a slurry reactor and others. If necessary, the catalyst may be recovered from the reactor, regenerated by drying and calcination, and recycled.
The reactivity of the catalyst of the present invention generally increases as the pH of the wastewater decreases. However, since the active ingredient of the catalyst may be leached out at a strongly acidic condition, the pH of the wastewater may preferably be adjusted to 3 or higher, more preferably to 6 or higher.
It is an important characteristic of the present invention that the wastewater treatment can be conducted effectively at ambient temperature and pressure, although the inventive process may be made more effective by conducting the inventive wastewater treatment at a higher temperature as the process allows. The inventive process may be preferably conducted at a temperature below 100xc2x0 C., more preferably below 50xc2x0 C.